Microelectrical-mechanical systems (MEMS) are miniature mechanical devices intended to perform non-electronic functions such as sensing or actuation. These devices are typically built from silicon using lithographic techniques borrowed from the semiconductor industry. Some examples of these devices are silicon pressure sensors and silicon accelerometers. Other manufacturing methods have been developed such as microembossing, stamping, microinjection molding, precision machining, and the like. These are typically used to build devices from non-silicon materials such as polymer or metal, for applications where silicon is not an appropriate material. Examples of such devices include microfluidic devices, biochips and optical devices. However, almost all micromachined devices must eventually be placed in a protective housing so that electrical connections can be made to the devices, and to protect the devices. This is troublesome for MEMS devices because they are fragile and so extreme care must be taken to move them from their fabricated substrates (e.g., wafers) to micro-electronic packages. It is well known that 60%-80% of the final cost for a MEMS device is from the costs associated with packaging.
The use of silicon for MEMS microfabrication has its roots in the successes of the semiconductor industry. Early MEMS designers in the 1980's looked to the semiconductor industry as a model for building small devices. Other manufacturing industries, such as precision machining, printed circuit board manufacturing, and microelectronic packaging did not have the manufacturing sophistication needed to produce devices with feature sizes in the few microns. In current times, however, these non-semiconductor industries have developed highly sophisticated tooling needed to do high precision manufacturing. These industries are now in an ideal position to take on the job of manufacturing MEMS devices.
There are at least four major manufacturing steps needed to make a final electronic product. These are:
1. Semiconductor manufacturing: A semiconductor manufacturer builds microcircuits on semiconductor material such as silicon (“microchips”).
2. Package base manufacturing: A substrate or lead frame manufacturer builds thin mechanical base structures for the chips. These can be laminate structures (“laminates”) or single precision cut layers of metal foil (“lead frames”).
3. Packaging: A packaging manufacturer assembles the chips on the base structures, makes electrical attachments, then puts a protective covering on them (“package”).
4. Printed circuit board manufacturing: A printed circuit board manufacturer makes a multilayer electrical laminate (“printed circuit board”) then takes assemblies and bonds packaged chips on the laminate to produce a final part (“printed circuit”).
For the purpose of this discussion, we will identify the last three manufacturing steps as “post semiconductor manufacturing” or PSM.
While MEMS devices have been built using semiconductor manufacturing techniques, little work has been done to demonstrate fabrication of MEMS using the three PSM techniques described above. There are several advantages that could be realized if MEMS devices were built using PSM techniques instead of the semiconductor approach. These are 1. Cheaper manufacturing: Non-semiconductor manufacturing is much cheaper than semiconductor manufacturing.
2. Better materials selection: Post-semiconductor manufacturing allows many more materials and to be included in the manufacturing process, including low temperature materials such as polymers.
3. Easier integration: Post-semiconductor manufacturing provides more flexible methods for manufacturing.
4. More variety: More materials and more manufacturing options yields a greater number of devices that can be designed and developed. Silicon is very limited in its uses.
5. Easier packaging: Since devices are built in packaging materials, using packaging techniques, packaging is easier to design.
Although MEMS devices can be built using manufacturing techniques that come from the PSM fields, little work is done in that area today. Thus it is desirable to provide methods for producing 3-D structures and free-standing structures using PSM techniques.